(1) Field of the Invention
The present invention relates to an air-fuel ratio control apparatus for a multiple cylinder engine.
(2) Description of the Related Art
Generally, in a gasoline engine, feedback compensation is carried out in which the fuel injection quantity is increased or decreased according to an output signal from an air-fuel ratio sensor provided in an exhaust pipe. Particularly in an engine system provided with a three-way catalyst and an O2 sensor, the fuel injection quantity is increased or decreased according to an output signal from an O2 sensor, and the air-fuel ratio of exhaust gas is maintained at around a stoichiometric air-fuel ratio to raise the purification rate of the three-way catalyst, such that exhaust gas can be purified to keep the exhaust gas exhausted from a tail pipe clean.
In a multiple cylinder engine, in the case where there are variations in exhaust air-fuel ratio among cylinders, gas which is rich or lean is emitted from each cylinder even if the average exhaust air-fuel ratio of all the cylinders can be maintained at a stoichiometric air-fuel ratio. If exhaust gases from the respective cylinders cannot be sufficiently mixed in an exhaust pipe, a large quantity of HC and CO passes through a three-way catalyst when the air-fuel ratio is rich, and on the other hand, a large quantity of NOX passes through the three-way catalyst when the air-fuel ratio is lean, and therefore, the HC, CO, and NOX cannot be effectively purified.
To address this problem, a technique according to which the fuel injection quantity is compensated for by estimating variations in exhaust air-fuel ratio among cylinders according to a value detected by an air-fuel ratio sensor provided upstream of a three-way catalyst (refer to Japanese Laid-Open Patent Publication (Kokai) No. 2001-82221, for example).
According to this technique, the exhaust air-fuel ratios of the respective cylinders are separately estimated so that exhaust gas can be purified with no variations in exhaust air-fuel ratio among the cylinders.
By the way, according to the above conventional technique, a crank angle at which the concentration of exhaust gas from one of the cylinders becomes dominative around the O2 sensor is obtained in advance by experiments or calculations, and the degree at which exhaust gas is rich or lean is detected independently for each cylinder according to the crank angle, and a deviation between the detection result and a stoichiometric air-fuel ratio are fed back, i.e., variations in exhaust air-fuel ratio are corrected by evaluating absolute values of the respective cylinders.
According to the above conventional technique, however, it is necessary to consider a response delay from exhaust valves of the respective cylinders to the O2 sensor, and in particular, if an exhaust pipe is not uniform in length, complicated model programming, experiments, and so forth for determining a crank angle at which the concentration of exhaust gas from one of the cylinders becomes dominative must be conducted to accurately recognize a response delay of each cylinder.
Namely, according to the above conventional technique, although variations in exhaust air-fuel ratio can be reduced, there is still the problem that it is impossible to provide control to easily reduce variations in exhaust air-fuel ratio.
It is therefore an object of the present invention to provide an air-fuel ratio control apparatus for a multiple cylinder engine, which is capable of correcting variations in exhaust air-fuel ratio among cylinders in a relatively easy way.